Wave filter



Nov. 9, 1954 Filed Jan. 4, l19:51

J. P. KINZER El' AL WAVE FILTER 2 Sheets-Sheet l J. n x/NzER /NVE/vroes= R. t mns/VALL l La. :mso/v ATTORNEY United States Patent O WAVE FILTER John P. Kinzer, Ridgefield, and Robert W. Marshall, Summit, N. J., and Ira G. Wilson, New York, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 4, 1951, Serial No. 204,352 28 Claims. (Cl. S33-73) This invention relates to frequency-selective electrical devices and more particularly to a wave filter suitable for operation in the microwave region.

A principal object of the invention is to improve the selectivity of a microwave filter.

Another object is to maintain the width of the transmission band of a microwave filter substantially constant while adjusting the mid-band frequency.

Further objects of the invention are to minimize the insertion loss and the distortion within the transmission band of a microwave filter.

In a microwave lter employing a cavity resonator which is resonant for a selected operating mode at the mid-band frequency the response of the resonator to unwanted modes often introduces deep valleys in the transmission loss characteristic outside of the pass band, thus greatly reducing the selectivity of the filter. In accordance with one feature of the present invention this difficulty is largely eliminated by constructing a microwave filter of two or more differently proportioned, electromagnetically coupled, cylindrical cavity resonators. Input coupling means are provided for introducing electromagnetic waves into one of the resonators, and output coupling means for extracting the filtered energy from another of the resonators. The coupling means may, for example, be orifices in the walls of the resonators.

Each of the resonators is resonant at approximately the mid-band frequency for a selected operating mode, which is not necessarily the same for all of the resonators. The resonators differ from one another in diameter, and consequently in length, by an amount sufiicient to prevent any two of the resonators from being resonant at the same frequency within or near the transmission band of the filter for any mode other than the selected mode or modes. That is, the frequencies of the undesired responses are staggered so that they will not coincide in any two of the resonators. Consequently, a dip in the loss characteristic due to an extraneous response in one resonator will be effectively filled in by the loss contributed by another of the resonators, thus providing a sustained suppression characteristic for the filter.

The deleterious effects of undesired modes may be further reduced by proper choices of the operating mode or modes, the internal diameters and lengths of the resonators, and the circumferential locations of the input and output coupling means. The transmission loss in the pass band is minimized by making the interior surfaces ofthe resonators highly conductive and by properly choosing the axial locations, shapes and sizes of the orifices. The distortion in the band is minimized by providing an approximately constant mode shape factor for each resonator and by properly choosing the sizes, shapes and locations of the orifices.

he mid-band frequency of the filter may be made adjustable by providing for each resonator an end plate which is movable longitudinally to change the'effective internal length of the cavity. In accordance with another feature of the invention, the width of the band is maintained substantially constant at all settings of the mid-band frequency by proper choices of the axial or longitudinal locations and the sizes and shapes of the coupling orifices.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings, of which:

Fig. 1 is a perspective view of a microwave filter in ice accordance with the invention comprising two coupled cylindrical cavity resonators;

Fig.r 2 is a horizontal sectional View of the filter, taken at the plane 2-2 in the direction of the arrows as shown in Fig. 3;

Fig. 3 is a fragmentary view of a section taken at a vertical plane through the longitudinal axis of the resonator shown at the let't in Fig. 1; and

Fig. 4 is a typical transmission loss-frequency characteristic obtainable with the filter. n

The embodiment of the microwave filter in accordance with the invention shown in Figs. l, 2 and 3 comprises two adjacent, electromagnetically coupled cavity resonators 5 and 6, in the form of right circular cylinders, each provided with input coupling means and output coupling means. As indicated by the arrow 7, electromagnetic wave energy from some suitable source, not shown, is introduced into the resonator 5 through the input wave guide 8 and a rectangular input orifice 9 in the side wall of the cylinder.

The output orifice 11 for the resonator 5 and the input orice 12 for the resonator 6 are circular and of the same diameter. The resonators 5 and 6 are so positioned that their longitudinal axes are parallel and the orifices 11 and 12 are substantially adjoining, thus forming the intercavity coupling. The lower, or fixed ends of the resonators may or may not lie in the same plane. After being filtered, the energy is extracted from the resonator 6 through the output coupling orifice 13 in the side wall of the cylinder and conducted down the output wave guide 14, as indicated by the arow 15, to a suitable load, not shown. The source and load each preferably match the characteristic impedance of the associated wave guide.

Each of the cylinders 5 and 6 may, for example, be spun, drawn or turned from some suitable metal such as brass or aluminum. In order to insure that the lower end plates of the cylinders 5 and 6 are substantially perpendicular to the longitudinal axes, leveling means may be provided, if required. In order to obtain a high quality factor, Q, and thus minimize the loss in the pass band, all interior surfaces of the resonators are preferably plated with a metal of high conductivity, such as silver. The cylinders are held together and in proper alignment by means of the bosses 16 and 17, which may be hard-soldered or brazed to the cylinders and milled to provide a plane surface 18 where they meet. In order to facilitate alignment, pins such as 19 and 20 project from the boss 17 and fit into corresponding holes such as 21 and 22 in the boss 16. The bosses may be held to` gether by soldering or brazing, or by means of screws or bolts, not shown.

The internal length, and therefore the resonant frequency f, of each of the cylindrical resonators 5 and 6 is made adjustable by means of a movable circular end plate 23 which has a diameter somewhat less than the inner diameter of the cylinder, thus providing a peripheral gap for suppressing extraneous modes having a radial component of the electric field vector. The plate 23 is secured to the end of a piston rod 24 which is free to move axially in the bearing 25 but prevented from rotation by a key 26 carried by the bearing 25 and fitting into a keyway 27 in the rod 24. Axial adjustment of the rod 24 and the end plate 23 is made by means of a micrometer head comprising an externally threaded inner tube 29 and an internally threaded cylindrical cap 30. A helical spring 31 acting against a collar 32 and a pin 33 holds the upper end of the rod 24 against a ball bearing 34 seated in the cap 30. -The position of the end plate 23 within the cylinder, and therefore the resonant frequency f of the resonator, is indicated by a circumferential scale 35 on the outside of the cap 30 and a vertical scale 36 secured to the cylinder.

The equivalent electrical circuit of the microwave filter described above comprises two inductively coupled circuits each tuned to resonate at the mid-band frequency f of the filter. The coupling coefficient is chosen to provide a band-pass characteristic with minimum loss and distortion within the transmission band. The design requirements are usually given in terms of the mid-band frequency f, the width of the transmission'band, the allowable distortion in the band, and the minimum discrimination between transmitted and suppressed frequencies. kThese requirements nmay "be transiatedinto the resonant frequency and workingq'uality' factor- Qv of the cavity resonators,l and the' requiredl coupling coefficient for the llter, -by'the application of well-known circuit theory such as is presented, for example,in paragraph 5 of 'section 3 of l'fermansiadio- Engineers Handbook, First-Edition, 'published-in 1943"by the McGraw-Hill Book Company, Incorporated, New York.

In the embodiment-disclosedherein, by way of example only, the requirements cai-led for a--substantiallyI constant four-megacyc'le-pass-baridfiiiiththe inici-band frequency f adjustable: from 7800 ytol'GO-rnegacyclesand distortion inithe bandi-not exceeding-Oll-decibel. The mid-band frequency rangeand thefdiscriinination requirement control the choice =ofl1theop'eiatingimode to be employed'in each of the resonatorsyand also the diameters and ranges of 'internal lengthsLLi fand Lz'of the resonators. The transverse-electric mode-designated TE in, in accordance with standardfnotatiomfis generally to be preferred. The third index number n, which'designates the number of half wavelength variations of the field along the axis of -the-cyli'ndenFmay be any integer. VIn this case, `the TE 012 mode was chosen foreach lof theresonators 5 and 6 because it hasia-fhighvalue'of -Q and its eld configuration ysimplifies themechanical design.

The selection of 'the lrequired internal diametersand lengths Li-and-Lz 'ofthe cylindrical resonators S and 6 ispreferably-'baseduponfa-study of appropriate mode charts, such-as are described at length in a paper by I. G. Wilson, C. W. Schramm, and J. P; Kinzer entitled High Q Resonant Cavities for lMicrowave Testing, in the Bell System Technical Journal, vol. XXV, No. 3, July 1946, u

pages `408 through 434. Additional pertinentl information may be found in a paper by 1.1P. Kinzer and I. G. Wilson entitled Some Results on Cylindrical Cavity Resonators, rinthe Bell'System Technical Journal, vol. XXVI, No.3, July 1947-pages 410 through 445 and in the book sRadar Systems` and Components by members of the Technical-Staff 'of Bell Telephone Laboratories, published Vin 1949 by theD. Van Nostrand Company, New York. 'Brieflyya study of the mode chart permits the choiceofian'operatingarea'in which the required band coveragemay be achieved'with the least amount of interference fromextrane'ousmodes. The selectivity in allcases is 1related tothe geometry of the cavity and the mode of oscillation. The range of ratios of the diameter to 'the length yforl each resonator should be such as to provide-an approximately constant mode shape factor throughout the tuning range, iny order to minimize distortion inthe transmission band. The mode shape factor maybe expressed as Q/h, where Q isthe unloaded quality factor for the resonator, is the skin depth for'the innersurface, and )wis thefree-spae wavelength.

Inaccordance withanV important feature ofthe invention the-discrimination of'vthev lter is greatly improved .by making the insidediametersl Diind D2 of the cylindrical resonators S--and 6 ysuiiciently-diterent to insure that the resonant frequencies of vunwanted modes which occur -in or near the transmission bandofthe'lter will be different in the two resonators, even -though vthe resonators have the same resonantfrequency f for the operating mode TE 012. This eiect can be demonstrated on two appropriate transparent mode charts by placing one over the other and displacing one with respectdo the other. However, the permissible difference between 'the diameters is usuallyli'rnitedbyfthc 'de'sii' to 'ob't'a'in a maximum tuning range. The choice of 'the diameters and the choice of the operating area are also influenced by those unwanted modesinost 'troublesome to Vsuppress and kthe relation of 'these modes'to'the operating mode. In the example under 'eonside'ratiom the inside diameter D1 of the-resonator Sis 2.42 inches'and the inside diameter D2 of the lresonator 6"'is 2.37 inches,"giving a difference 'ofa little nore "th'ari two percen't.

In accordance with another important feature'of the invention, the'iocatio'ns of the orifices9, 11, v12 and 13 are-selected to achieve, as nearly as'possible, maximum discrimination, minimum' lossv and' distortion throughout thetransmission band, and -a;constant bandwidth for all mid-band frequencies within the tuning range. In each oftheresonators 5 andoL the circumferential angle between' the input orifice andthe output orifice is sochosen that interference from unwanted modes having certain 'typesof'glar'peridicty 'isriin'im'i'zed 'In general, these angles are chosen to minimize interference by modes having radial components of the electric iield in the neighborhood of the angular coordinate of the center of the output orifice. The choice-of angle thus depends upon the particular modes to be suppressed. As shown in Fig. 2, for the iilterhere considered the angle @l between the input orifice 9 and the output orifice 11 of the resonator 5; measured-in a clockwise direction, is approximately degrees, andthe corresponding-.angle 1D, between lthe orifices` 12 aridl 13 of lthe resonatori vis approximately degrees.

The longitudinal locations ofthe orifices 9, 11, 12 and 13 are dictated" vprincipallybythe'desire Ato' maintain a constant bandwidth at'all 'mid-band settings of the lter. It is found from an analysis'of orifice equations such as are presented in Table IV of the above-cited paper in the July; 1946 lBellf-System Techni'cali 'Journal Ith'at ithe vbandwidth f 'will- :have-la minimum variation 'over A lthe ituning range if, for the inputforiii'ce9`-and-itheqoutput orifice 13, the distanee'Aifromaheeentenef.-therice tothe-stirred end of the respective cylinder --isflfsol1fchosen 'tthatfthe quantity ne? LL .l 1ML is kept as nearly constant as po's's'ibleffor all`values`ofthe internal length' L of fthe cylinder. The symbol x'represents thetree-spacewaveiength'attheinid=band^operating frequencyv 12kg' is"thewav'elen'gthf at thes'arne frequency inthe wave' guide associated with the priica'and wis 'the index defined above. Inf'the ilter' under `lconsideration, nis two. Thelongitudinalrspacings of theori'ces9'and 13 f'rnnA thei 'respecti'vej tijredtends ofi the "resonatorsF'S and 6" determined 4by vthe fore-going'considerations arenot necessarily the'- sarnejin bother-the 'resenato'rs.

LSimilar considerations show that; fdr 'maximuml constancy or the bandwidth; the distance "1B horn `the center of'theoufpucorince 1'1-oftheresonator 5 toithe fixed-end thereof'and'f-theidistance C from the center nfld-ie input orifice 12pt fthe 'resonator 6 to-'its fixedl end` a're chosento make the quantity as nearly y constant as possible over the tuning range, where-L1 and L are 4the internal 'lengthsof .the ,resonators 5 and 6, respecti yely. -Since-the optimum valuesof-the distances Band-Chchtpseninthis way.rna y notrbe-vthe same, 1 it followsrthat .the lower endsofV the cylinders Sand 6 Amaynot be-in-thesame plane.

:Each Iot` the distances'A,-,B and@ will beapproximately equal toL/Zn. I n'ltheL-iilterunder-consideration the variationrinthe bandwidth was :held to-limits oflesst-han l1 per cent yby making the distance .A for.' the inputforiiice 9 equal to 1,275 f ches,` the corresponding.distancelffor the output 4oriceV 13 equal to` l`3'40 .inc hes, .andltliedistances B andC each equall to 11215' inches. A'. typical transmission loss-frequency characteristic for one` setting of the `t`1lteris`shown `in` Eig. 2i; where the loss .inl decibels is plotted against 'the diference ofthelfrequency-i'n megacycles rro'mthe'm'id-b'and frequencyff. 4The ilter" has a low andsuhstantiallycbnsfant less within 'the transmission band of r.approxir nately 'foun `'rnegacycles' and a"'steep1y rising suppression "oljiaract'eristiconieither side ofthe band, with noy discernible dips causedfbyminterferingmdes over the range from 3() rneg'acycles -below Y'to 30 megaoyeles above f.

The required diameterof theoril'ces 11"ancl112" rr ray be 'calculated to `a"'1"st:ipproximationffrom formulas found ivin y"the `referenties 'cit'ed above. However, `tlese formulas assum-that the orifice' is "locatd infa'fveryl thin wall. Gene'rally"the'walls'vofpractical resonators, such as" S'and, are thick enough 'td frequireil thatltheiadiameter off theforice'ibe correeteditovallowfor t-he thickness of'ithe Wall. 'Bhe-'correctiontoifbe'iapplied fmay be found by experiment.

1. 1AI rnicrowave`iilter {comprising two cylindrical cavity resonators -eaeh resonant` vforza selected: CIE =0 1n operating modeat .approximately At 'e mid-band frequency 'offthe nner; iiiean' fof' adjusting the ietive' electrical lengths of said resonators to adjust said frequency over a range, and intercavity coupling means having a coupling coefficient such .that a desired band-pass characteristicis imparted to vthe filter, said resonators having different inside diameters so chosen that, for all yfrequency adjustments within said range, rallresonancesfor non-selected modes falling within or near the transmission band of the filter occur at respectively different frequencies in each of said resonators, thereby improving fthe: discrimination ofthefilter.

2. A filter in r.accordance with. claim 1 in which the opeating mode of each of said resonators. is theTE 012 mo e.

3. A filter in accordance with claim l in which for each of said resonators the range of ratios of said diameter to said length is chosen to provide an approximately constant mode shape factor throughout the adjustable range.

4. A microwave filter comprising two cylindrical cavity resonators each resonant for a selected TE Oln operating mode at the mid-band frequency f of the filter, an input coupling device, an output coupling device, and an adjustable end plate associated with each of said resonators, means including an input wave guide for introducing electromagnetic wave energy into one of said resonators through its input coupling device, means for connecting said output coupling device of said one resonator and said input coupling device of the other of said resonators, and means including an output wave guide for extracting filtered energy from said other resonator through its output coupling device, the center of said input coupling device in said one resonator and the center of said output coupling device in said other resonator each being spaced from the end opposite said adjustable end plate a distance A so chosen that the quantity A sin2 A L is kept as nearly constant as possible for all positions of said end plate, where A is the free-space wavelength at the frequency f, Ag is the wavelength at the frequency f in the wave guide associated with the coupling device, L is the internal length of the resonator, and n is the number of half wavelength Variations of the field along the longitudinal axis of the resonator for the operating mode of the resonator.

5. A filter in accordance with claim 4 in which said resonators differ from each other in inside diameter to prevent both of said resonators from being resonant at the same frequency Within or near the transmission band of the filter for any mode other than the selected operating mode or modes of said resonators.

6. A filter in accordance with claim 4 in which the center of said output coupling device in said one-resonator is spaced a distance B from the end of the resonator opposite said adjustable end plate and the center of said input coupling device in said other resonator is spaced a distance C from the end of the resonator opposite said adjustable end plate, said distances being chosen to make the quantity sin sin L1 L2 LIL;

as nearly constant as possible for all positions of said end plates, where L1 and L2 are the internal length, respectively, of said resonators.

7. A filter in accordance with claim 6 in which said resonators differ from each other n insidediameter to prevent both of said resonators from being resonant at the same freqeuncy within or near the transmission band of the filter for any other than the selected operating mode or modes of said resonators.

8. A filter in accordance with claim 4 in which in each of said resonators the circumferential angle between said input coupling device and said output coupling device is chosen to minimize interference by modes having radial components of the electric field in the neighborhood of the angular coordinate of the center of said output coupling device.

9. A filter in accordance with claim 8 in which said resonators differ from each other in inside diameter to prevent both of said resonators from being resonant at the same frequency within or near the transmission band ofthe filter for any other than.the selected operating mode or modes of said resonators.

.10. A microwave filter comprising two cylindrical cavity resonators, each. of said resonators being resonant for a selected TE Oln operating mode at approximately the mid-band frequency of the filterincluding an adjustable end plate for adjusting said frequency over a range, and having an input lorifice and an output. orifice, and means for connecting said output orifice of one of said resonators and said input orifice of the other of said resonators to constitute an intercavity coupling having a coupling coefficient such that a desired band-pass characteristic is imparted to the filter, said resonators having different inside diameters so chosen that, for all frequency adjustments within said range, all resonances for non-selected modes falling within or near the transmission band of the filter occur at respectively different frequencies in each of said resonators, thereby improving the discrimination of the filter.

l1. A filter in accordance with claim 10 in which the ope'ating mode of each of said resonators is the TE 012 mo e.

l2. A filter in accordance with claim 10 which includes means for indicating the internal lengths of said resonators. f

13. A filter in accordance with claim 10 in which for each of said resonators the circumferential angle between said input orifice and said output orifice is chosen for maximum discrimination between transmitted and suppressed frequencies.

14. A filter in accordance with claim 10 in which the size, shape, and axial location of each of said orifices are chosen for minimum loss within the transmission band of the filter.

15. A filter in accordance with claim 10 in which the size, shape, and location of each of said orifices are chosen to minimize the distortion in the transmission band of the lter.

16. A lter in accordance with claim 10 in which for each of said resonators the range of ratios of said diameter to the internal length of the resonator is chosen to provide an approximately constant mode shape factor for all positions of said end plate.

17. A filter in accordance with claim 10 in which for each of said resonators the shapes, sizes and axial locations of said orifices are chosen to provide maximum constancy of bandwidth for all position of said end plate.

18. A band-pass Wave filter adapted for operation in the microwave region comprising two cylindrical cavity resonators, input coupling means associated with one of said resonators, output coupling means associated with the other of said resonators, means for electromagnetically coupling said resonators, and a movable end plate associated with each of said resonators, both of said resonators being resonant at substantially the same frequency for a selected TE Oln operating mode but differing from each other in inside diameter, the diameters of said resonators and the circumferential locations of said coupling means being chosen to minimize the deleterious effects of undesired modes, and the axial.

locations and coupling coefficients of said coupling means being chosen to provide the desired width of transmission band and to maintain said width as nearly constant as possible for all positions of said end plates.

19. A filter in accordance with claim 18 in which the opecxlating mode of each of said resonators is the TE 012 mo e.

20. A filter in accordance with claim 1 in which the inside diameters of said resonators differ by at least one per cent.

21. A filter in accordance with claim 1 in which the inside diameters of said resonators differ by at least two per cent.

22. A filter in accordance with claim 1 in which the inside diameters of said resonators differ by approximately two per cent.

23. A filter in accordance with claim l which includes means for indicating the internal lengths of said resonators.

24. A filter in accordance with claim 4 which includes means for indicating the internal lengths of said resonators.

25. A filter in accordance with claim 4 in which the inside diameters of said resonators differ by approximately two per cent.

26. A lter in accordance with claim 10 in which the References Gite'inthdileofltmpment UNITEDJ ISTTES PTENTS 

